Circuit and method for contact pad isolation

ABSTRACT

A circuit is provided to isolate a contact pad from a logic circuit of a die once the contact pad is no longer needed. This circuit can take many forms including a CMOS multiplexer controlled by a fuse or anti-fuse, an NMOS or PMOS pass gate controlled by a fuse or anti-fuse, or even a fusible link which is severed to effect isolation. Additionally, a circuit is provided that switchably isolates one of two contact pads from a logic circuit.

This application is a divisional of U.S. patent application Ser. No.09/023,639, filed Feb. 13, 1998.

TECHNICAL FIELD

This invention relates generally to electronic devices and, morespecifically, to a circuit and method for isolating a contact pad from alogic circuit.

BACKGROUND OF THE INVENTION

Processed semiconductor wafers typically comprise an array ofsubstantially isolated integrated circuitry locations, which aresubsequently separated to form semiconductor dies. In order to test theoperability of the integrated circuitry of a die location on a wafer, awafer probe card is applied to each die location. The wafer probe cardincludes a series of pins that are placed in physical contact with a dielocation's contact pads, which in turn connect to the die location'scircuitry. The pins apply voltages to the input contact pads and measurethe resulting output electrical signals from the output contact pads.However, the wafer probe card's pins may not be able to extend to all ofthe contact pads. As a result, it is necessary to provide accessibleredundant contact pads on the die location and couple them to particularlogic circuits.

An additional hardware limitation relevant to testing the die locationsis the spacing between the pins of the wafer probe card. Specifically,the pins may be spaced further apart than the contact pads in aparticular area of a die location. As a result, one contact pad in thatarea may not be serviceable by a pin. As a solution, prior art teachesproviding a redundant contact pad in another area of the die locationthat can be reached by a pin. This redundant pad is connected to thesame logic circuit as the unserviceable contact pad.

There may also be other reasons for including additional contact pads ona die. Regardless of the reasons, prior art allows these redundantcontact pads to remain connected to the logic circuit after they are nolonger needed. By remaining connected, these redundant contact padscontribute additional capacitance to their associated logic circuits andthereby degrade performance of the die.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a circuit for isolating acontact pad from a logic circuit. In a first exemplary embodiment, acomplementary metal-oxide semiconductor (CMOS) multiplexer connects aredundant pad to a logic circuit, wherein the CMOS multiplexer iscontrolled by a fuse. Programming the fuse disables the multiplexer andprevents the redundant contact pad from affecting the logic circuit.Thus, this embodiment has the advantage of removing a parasiticcomponent that might degrade performance of the logic circuit.

In a second exemplary embodiment, one fuse circuit controls severalmultiplexers, wherein each multiplexer services a separate logiccircuit. This embodiment offers the advantage of reducing capacitance ofseveral logic circuits while simultaneously conserving the die spaceneeded to do so.

In a third exemplary embodiment, one fuse circuit controls twomultiplexers, wherein both multiplexers service the same logic circuit.In addition to interposing a first multiplexer between the redundantcontact pad and the logic circuit, a second multiplexer is interposedbetween a main contact pad and the logic circuit. Further, this secondmultiplexer is configured to operate conversely to the firstmultiplexer. Thus, before the fuse is programmed, only the redundantcontact pad is in electrical communication with the logic circuit. Afterthe fuse is programmed, only the main contact pad is in electricalcommunication with the logic circuit. The advantage offered by thisembodiment is that, while one contact pad is being used, the othercontact pad does not contribute additional capacitance.

A fourth exemplary embodiment combines the features described in thesecond and third exemplary embodiments. Thus, not only does one fusecontrol the electrical communication of several logic circuits, but thefuse also controls which contact pad can be used with each logiccircuit. Accordingly, this embodiment combines the advantages found inthe second and third embodiments. A fifth embodiment achieves the sameadvantages discussed above using an anti-fuse in place of the fuse. Inaddition, all of the embodiments listed above providecapacitance-reducing advantages while avoiding accidental programming ofthe fuse due to an ESD event.

Moreover, a sixth exemplary embodiment replaces the fuse controlledmultiplexer with the fuse itself for linking the redundant contact padwith the logic circuit. In doing so, this embodiment offers all of thecapacitance-reducing advantages of the embodiments discussed above andtakes up less die space.

In a seventh exemplary embodiment, an isolation circuit is used during atest mode to connect a logic circuit to a no-connect pin on anintegrated device, thereby providing the advantage of having anadditional access point for testing the integrated device. Once the testmode has ended, the fusing element is programmed and the no-connect pinelectrically disconnects from the logic circuit.

In an eighth exemplary embodiment, a die is provided having two groupsof contact pads, wherein each group is configured to accommodate adifferent lead frame. One contact pad from each group is connected to aparticular logic circuit. An isolation circuit similar to the fourthexemplary embodiment is provided to regulate electrical communicationwith the contact pads. Specifically, in an unprogrammed state, theisolation circuit electrically isolates the second group of contact padsfrom the logic circuits. The first group remains in electricalcommunication with the logic circuits and may accommodate an appropriatelead frame. If, on the other hand, a lead frame is chosen that iscompatible with the second group of contact pads, then the entire firstgroup 64 can be isolated in a single programming step that also servesto enable communication between the entire second group 66 and the logiccircuits. This embodiment has the advantage of providing a die that iscompatible with two different types of lead frames. In addition, theadaptation requires at most one programming step. As a furtheradvantage, this embodiment restricts additional capacitance fromunneeded contact pads once the appropriate lead frame has beendetermined.

A ninth exemplary embodiment is configured in a manner similar to theeighth embodiment. Rather than including one all-encompassing isolationcircuit, however, this embodiment includes several isolationcircuits—one for each logic circuit. Each isolation circuit resemblesthe third exemplary embodiment in that the isolation circuit can be usedto determine which contact pad communicates with the logiccircuit—either the pad from the first group or the pad from the secondgroup. By allowing a programming choice for each logic circuit, thisembodiment provides a die that can adapt to other lead frames inaddition to the two lead frames addressed in the eighth embodiment.Accordingly this embodiment also restricts additional capacitance fromunneeded contact pads once the appropriate lead frame has beendetermined.

In addition to these circuit embodiments, the present inventionencompases various methods for achieving these advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a wafer probe card superimposed over a die.

FIG. 2 demonstrates a circuit used in the prior art for testing a logiccircuit on a die.

FIG. 3 illustrates a first exemplary embodiment of the presentinvention.

FIG. 4 illustrates a second exemplary embodiment of the presentinvention.

FIG. 5 illustrates a third exemplary embodiment of the presentinvention.

FIG. 6 illustrates a fourth exemplary embodiment of the presentinvention.

FIG. 7 portrays a fifth exemplary embodiment of the present invention.

FIG. 8 depicts a sixth exemplary embodiment of the present invention.

FIG. 9 depicts a lead frame having a conductive lead configuration andaccommodating a plurality of dies.

FIG. 10a is a partial pin-out diagram of a typical integrated devicethat exists in the prior art.

FIG. 10b demonstrates a seventh exemplary embodiment of the presentinvention.

FIG. 11 displays an eighth exemplary embodiment of the presentinvention.

FIG. 12 displays a ninth exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates the top view of a wafer probe card 10 having a seriesof pins 12 extending from two sides of the wafer probe card 10. In orderto test a particular die 14 on a wafer, this wafer probe card 10 isplaced over the die 14. The die 14 includes a plurality of contact pads16. For purposes of this application, a contact pad is defined toinclude any conductive surface configured to permit temporary orpermanent electrical communication with a circuit or node. Duringtesting, the pins 12 of the wafer probe card 10 are in communicationwith nearby contact pads 16. Given the configuration of the wafer probecard 10, however, the pins 12 may not be able to reach contact pads 16on certain areas of the die 14.

FIG. 2 demonstrates the solution in the prior art for this problem. Ifthe contact pad 16 for a logic circuit 18 cannot be accessed by thewafer probe card 10, then a redundant contact pad 20 is provided in amore accessible location and coupled to the logic circuit 18. Aftertesting, the original contact pad 16 is once again used to access thelogic circuit 18. The redundant contact pad 20, however, also remainscoupled to the logic circuit 18 and, as described above, may adverselyaffect the performance of the logic circuit in particular and the entiredie in general.

FIG. 3 illustrates one embodiment of the current invention that solvesthe problem remaining in the prior art solution. An isolation circuit 22is electrically interposed between the redundant contact pad 20 and thelogic circuit 18 in order to regulate electrical communicationtherebetween. The isolation circuit 22 in this embodiment comprises ap-channel long L device 24 having a source coupled to a potential node26. The potential node 26 is configured to accept a voltage source. Thep-channel long L device 24 also has a drain coupled to a signal node 28.The gate of the long L device 24 is bled to ground, thereby allowingsignal node 28 to constantly receive a voltage signal from the potentialnode 26.

The signal node 28 carries the voltage signal through a pathway leadingto ground, but that pathway is interrupted by a fuse 30. Moreover, thesignal node 28 is coupled to a first inverter 32. The output of thefirst inverter 32 connects to the gate of an n-channel transistor Q1,which is a component of a multiplexer 80 electrically interposed betweenthe redundant contact pad 20 and the logic circuit 18. In addition, theoutput of the first inverter 32 serves as the input for a secondinverter 34. This second inverter 34 connects to the gate of a p-channeltransistor Q2, which is another component of the multiplexer 80.

In operation, the potential node 26, the p-channel long L device 24, thesignal node 28, and the fuse 30 cooperate to determine the drive stateof the multiplexer 80. The fuse 30 is initially intact and providesgrounding communication for the signal node 28. Because the signal node28 is grounded, a low voltage signal, or “logic 0,” is generated andcarried to the first inverter 32. Accordingly, the first inverteroutputs a high voltage signal, or “logic 1.” The high signal drives then-channel transistor Q1. The high signal also serves as input to thesecond inverter 34, and the resulting low signal drives the p-channeltransistor Q2. With transistors Q1 and Q2 on, a range of signals may betransmitted between the redundant contact pad 20 and the logic circuit18.

The redundant contact pad 20 can then be isolated by programming or“blowing” the fuse 30. With fuse 30 blown, the signal node 28 no longerhas a direct path to ground. As a result, a high signal is sent in a newdirection—to the first inverter 32. The resulting low signal turns offthe n-channel transistor Q1. Further, the low signal is changed by thesecond inverter 34 to a high signal that turns off the p-channeltransistor Q2. With both transistors Q1 and Q2 off, electricalcommunication between the redundant contact pad 20 and the logic circuit18 is prevented.

Moreover, the potential node 26/signal node 28/fuse 30 configuration,hereinafter referred to as a “program circuit,” is not limited todriving only one multiplexery. As seen in FIG. 4, first inverter 32 andsecond inverter 34 can also be coupled to transistors Q1′ and Q2′ of asecond multiplexer 80′, wherein the second multiplexer 80′ iselectrically interposed between another logic circuit 18′ and anotherredundant contact pad 20′. As a result, this embodiment provides for theelectrical isolation of two redundant contact pads by blowing only onefuse. Contact pads 16 and 16′ maintain electrical communication withtheir respective logic circuits 18 and 18′. It follows that additionallogic circuits could be similarly accommodated.

In yet another embodiment illustrated in FIG. 5, a multiplexer 81 iselectrically interposed between contact pad 16 and the logic circuit 18.As with multiplexer 80, multiplexer 81 is comprises a p-channeltransistor Q3 and an n-channel transistor Q4. However, whereas the firstinverter 32 is coupled to the n-channel transistor Q1 of multiplexer 80,the first inverter 32 is instead coupled to the p-channel transistor Q3of multiplexer 81. Similarly, the second inverter 34 connects to thep-channel transistor Q2 in multiplexer 80 but drives the n-channeltransistor Q4 in multiplexer 81. By switching the driving signals inthis fashion, the initial signals that serve to turn on multiplexer 80also turn off multiplexer 81. Conversely, blowing the fuse, which turnsoff multiplexer 80, serves to turn on multiplexer 81.

Further, the embodiments depicted in FIGS. 4 and 5 could be combined sothat blowing one fuse 30 switches the communication arrangement for twoor more logic circuits. Thus, as demonstrated in FIG. 6, multiplexers80′ and 81′ are driven by the program circuit to allow electricalcommunication between logic circuit 18′ and redundant contact pad 20′,while at the same time electrically isolating contact pad 16′.Meanwhile, the same program circuit allows for electrical communicationbetween logic circuit 18 and redundant contact pad 20 and electricallyisolates contact pad 16. Blowing fuse 30 switches the electricalcommunication pathways for both logic circuits 18 and 18′.

FIG. 7 demonstrates another embodiment of the current invention. Theisolation circuit 22 has a similar configuration to the one in FIG. 3except that (1) the fuse 30 has been replaced with an anti-fuse 36; (2)the second inverter 34 now drives the n-channel transistor Q1; and (3)the first inverter 32 directly drives the p-channel transistor Q2. Giventhis configuration, the direct path from the signal node 28 to ground isinitially barred by the anti-fuse 36. Consequently, a high signal istransmitted to the first inverter 32. The low signal output drives thep-channel transistor Q2. The second inverter 34 turns this low signalinto a high signal in order to drive the n-channel transistor Q1. Withboth transistors Q1 and Q2 on, the redundant contact pad is fullycoupled to the logic circuit. Once the anti-fuse is programmed, however,the signal node 28 becomes grounded and a low signal is transmitted tothe first inverter 32, which sends a high turnoff signal to thep-channel transistor Q2. Moreover, this high signal is altered by thesecond inverter 34 so that a low signal turns off the n-channeltransistor Q1. With both transistors Q1 and Q2 off, the redundantcontact pad 20 is no longer in electrical communication with logiccircuit 18.

It can be appreciated that an anti-fuse 36 could replace the fuse manyof the embodiments of this invention. Accordingly the “program circuit”could include an anti-fuse.

An embodiment illustrated in FIG. 8 demonstrates that the isolationcircuit 22 can comprise the fuse 30 directly interposed between theredundant contact pad 20 and the logic circuit 18, wherein programmingthe fuse isolates the redundant contact pad 20. Programming can occur atthe completion of testing or at a stage in any other application whereisolation of a contact pad is beneficial. It should be noted that, whilethis embodiment conserves die space, embodiments such as those in FIGS.3 through 7 are better at preventing accidental programming due to anESD event.

If wafer testing indicates a likelihood that the wafer has a yield ofgood quality dies, the dies are separated from the wafer and undergo apackaging process. Many such processes involve attaching a die 14 to alead frame 42, such as one shown in FIG. 9, and using bond wires 44 toconnect the contact pads 16 to the conductive leads 46 of the lead frame42. The die/lead frame assembly may then be encased, with the outer endsof the conductive leads 46 remaining exposed to allow communication withexternal devices. However, some conductive leads may not be connected tothe contact pads of a die. Such a conductive lead is designated as a“no-connect” or “NC” pin, as demonstrated in the pin-out diagram of FIG.10a.

After assembly, a packaged device may then be subjected to furthertesting. FIG. 10b depicts an embodiment of the current invention thatmakes use of the no-connect pin 38 of the packaged die 14 for suchtesting. Prior to assembly, the die 14 is configured to include aredundant contact pad 20 coupled to a logic circuit 18 through anisolation circuit 22. Further, the no-connect pin 38 is connected to theredundant contact pad 20 by a bond wire 44. As a result, communicationwith the logic circuit 18 may be accomplished during testing of thedevice through the no-connect pin 38. Once testing is complete, theisolation circuit 22, which may comprise one of the configurationsdescribed above, is programmed, thereby halting communication betweenthe no-connect pin and the logic circuit.

Moreover, other embodiments of the current invention allow for isolatingan additional contact pad that is not necessarily a test-mode pad. Asshown in FIG. 11, isolation circuits can be used to allow a die to adaptto more than one lead frame configuration. FIG. 11 shows eight logiccircuits 48, 50, 52, 54, 56, 58, 60, and 62 coupled to a first group ofcontact pads 64 located on opposing sides 68, 70 of a die 14. Theseeight logic circuits are also coupled to a second group of contact pads66 extending along a center axis 76 of the die 14 between the betweenthe opposing sides 68, 70. An isolation circuit 22 is also provided. Inthis embodiment, the isolation circuit 22 resembles the one depicted inFIG. 6, where the isolation circuit 22 not only services more than onelogic circuit but also enables exclusive electrical communication withina logic circuit to be switched between two contact pads.

FIG. 11 further demonstrates that the first group of contact pads 64 isconfigured to accommodate a lead frame having conductive leads 72 thataddress the opposing sides 68 and 70 of the die 14. The second group ofcontact pads 66 will favorably receive a lead frame having conductiveleads 74 addressing internal portions of the die, such as those near thecenter axis 76. Thus, depending on the lead frame ultimately chosen, thecurrent invention allows for particular contact pads to be isolatedaccordingly. As in FIG. 6, the isolation circuit in FIG. 11 is assumedto be configured to turn on the transistors in multiplexers 80 when thefuse is intact. It should also be noted that multiplexers 80 areinterposed between the first group of contact pads 64 and theirrespective logic circuit. Further, multiplexers 81 are interposedbetween the second group of contact pads 66 and their respective logiccircuit. Thus, if the fuse 30 is not blown, then electricalcommunication with the logic circuits 48, 50, 52, 54, 56, 58, 60, and 62is achieved solely through the first group of contact pads 64. Should itbe determined to package the die 14 with a lead frame having conductiveleads 72, the fuse remains unprogrammed, the conductive leads 72 arewire bonded to that group, and the second group of contact pads 66remain isolated. If, however, a lead frame including conductive leads 74is to be packaged with the die 14, then by programming a single fuse 30,the second group of contact pads will be in electrical communicationwith the logic circuits 48, 50, 52, 54, 56, 58, 60, and 62. Moreover,the first group of contact pads 64, having been isolated due to blowingthe fuse, will not contribute additional capacitance to the circuitoperations.

The embodiment illustrated in FIG. 12 can accommodate still other leadframes, wherein only some of the contact pads of a group need to beisolated. While the logic circuit/contact pad layout in FIG. 12 issimilar to the configuration in FIG. 11, the isolation circuitry ispreferably more like the arrangement in FIG. 5. Furthermore, It would bebeneficial in this embodiment to use a plurality of isolation circuits22 in order to provide one fuse 30 for every contact pad pair associatedwith a logic circuit. Given this configuration, each fuse 30 can beprogrammed as needed to accommodate the lead frame. For example, thelead frame in FIG. 12 has some conductive leads 74 addressing internalportions of the die near the center axis 76, and the lead frame hasother conductive leads 72 that address opposing sides 68 and 70 of thedie 14. Therefore, only some of the contact pads in the first group 64should be isolated, as should some of the contact pads in the secondgroup 66. The embodiment in FIG. 12 allows this selectivity.

It would be a further benefit to associate a particular group of contactpads with multiplexers having the same initial state. For example,assuming that each contact pad in the first group 64 is respectivelycoupled to the multiplexer 80 of each isolation circuit 22, it followsthat the entire first group 64 is initially in electrical communicationwith the logic circuits 48, 50, 52, 54, 56, 58, 60, and 62. It alsofollows that the entire second group 66 is associated with themultiplexers 81 of the isolation circuits 22 and are therefore isolated.In order to accommodate the conductive leads 72, 74 illustrated in FIG.12, it is relatively easy, given contact pad/isolation circuitassociation, to determine that only the fuses 30 corresponding to logiccircuits 50, 52, 58, and 60 need to be blown.

In addition, one can appreciate that other lead frame adapterembodiments could use isolation circuits similar to those depicted inFIGS. 3, 7, and 8.

Finally, one of ordinary skill can appreciate that, although specificembodiments of this invention has been described for purposes ofillustration, various modifications can be made without departing fromthe spirit and scope of the invention. For example, concerning theembodiments discussed above that use a fuse, such a fuse could compriseone of various types of fuses, including a link fuse or a laser fuse.Alternatively, the fuse could be replaced by an anti-fuse with minorconfiguration changes. Moreover, embodiments such as those in FIG. 3through 7 using both a p-channel and an n-channel transistor as a linkcould be modified to use only one of the transistors. Accordingly, theinvention is not limited except as stated in the claims.

What is claimed is:
 1. A method of regulating electrical communicationwith a logic circuit coupled to a first communication terminal and asecond communication terminal, comprising: initially allowing electricalcommunication between said logic circuit and said first communicationterminal and between said logic circuit and said second communicationterminal; and subsequently preventing electrical communication betweensaid logic circuit and said second communication terminal.
 2. A methodof modifying electrical communication between a logic circuit and firstand second communication terminals, comprising: subjecting electricalcommunication between said logic circuit and said first communicationterminal to a multiplexing function; making said multiplexing functiondependent upon an operational state of a programming element; andrefraining from subjecting electrical communication between said logiccircuit and said second communication terminal to said multiplexingfunction.
 3. A method of regulating electrical communication with alogic circuit coupled to a first communication terminal and a secondcommunication terminal, comprising: allowing electrical communicationbetween said logic circuit and a selection of said first communicationterminal and said second communication terminal, wherein said firstcommunication terminal and said second communication terminal are ondifferent sides of a die; and optionally changing said selection.
 4. Themethod in claim 3, wherein allowing electrical communication furthercomprises: multiplexing electrical communication between said logiccircuit and said first communication terminal; and converselymultiplexing electrical communication between said logic circuit andsaid second communication terminal.
 5. The method in claim 3, whereinoptionally changing said selection comprises providing for oppositelymultiplexing electrical communication between said logic circuit andsaid first communication terminal and between said logic circuit andsaid second communication terminal.